The Carver and the Queen - Emma C. Fox

In the age of Imperial Russia, deep in the Ural Mountains, two teenagers plot their escape from serfdom and dream of a destiny beyond their birth. Petr, a castoff orphan, longs to make a name for himself as a sculptor of malachite, the Ural's most treasured gem. His best friend Lena harbors an even more impossible goal: to escape an arranged marriage to the cruel village bailiff and become an artist in her own right.

When their bid for freedom falls to pieces, Petr seeks help from the mythical Malachite Queen, only to find himself trapped in an underworld where godlike skill comes at the cost of his humanity. It's up to Lena to contrive a daring rescue before Petr is lost forever in a kingdom of stone.